An auxiliary power boost steering system having an emergency pump which operates with a first and a second pump has been known, for example, in the German Pat. No. 27 31 975 (U.S. Pat. No. 4,237,993). A two-pump operation is always desirable since at times heavy trucks become unsteerable because of the failure of the engine driven first pump. In this case, a vehicle may be safely pulled out of traffic while the vehicle is rolling, for example, with the second pump (emergency steering pump) driven by the gearing in vehicle movement. The known auxiliary power steering arrangement shows a so-called emergency valve which, in the case of a defect of the engine driven first pump, separates the latter from the hydraulic steering circuit and connects in the emergency steering pump. Whenever the engine driven first pump is fully operational, the emergency steering pump in the standby mode circulates a volume of oil with a certain idling pressure required for the maintenance of the hydraulic circuit. Since a failure of the engine driven first pump occurs only very rarely during operation time of a vehicle, the continuing standby operation of the vehicle driven emergency steering pump means a steady loss of power.